想请问一下这题统计问题 搞不太清楚在问什麽@@ Mary lives at O in the accompanying diagram and has four friends who live atA,B,C,D. One day Mary decides to go visiting, so she tosses a fair coin twice to decide which of the four to visit. Once at a friend's house, she will either return home or proceed to one of the two neighbors (such as O,A,C when atB) with each of the three possibility having probability 1/3. In this way Mary continues to visit friends until she returns home. A___________ B |＼ ／| 正方形(不好意思 画的很差orz) | ＼ ╱ | | ╳ | | ╱O ╲ | |／ ＼| D—————— C Let X be the number of times that Mary visits a friend, Y be the number of straight-line segment that Mary traverses/穿越/(including those leading to and from O) Which one of the following statement is true? A) f(x)=(2/3)*(1/3)^x-1 for x=1,2,3 B) f(y)=(3/4)^y-1 * (1/4) for y=2,3,4,5 C) f(x)=(3/4)^x-2 * (1/4) for x=1,2,3 D) f(y)=(2/3)^y-2 * (1/3) for y=2,3,4,5 X是Mary拜访朋友次数 Y我就搞不太懂是代表什麽@@ 可以问一下大家 这题大概是在问什麽吗??? 因为答案里的假设我也搞不太懂是为什麽@@" 为什麽是这样假设呢??? 谢谢大家的帮忙了!!!!! Orz --

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